This project involves the study of erythropoietin-responsive cells, cells that respond to colony-stimulating factor, pluripotent hematopoietic stem cells, and the interrelationships among them. Our working hypothesis is that these are essentially the same cells and can be defined by the presence or absence of receptors for the inducers of hematopoietic cell differentiation. We are studying the properties of the erythropoietin receptor of mouse cells and the role of interleukin 3 on the regulation of response to erythropoietin. (M)